During a month some questions have gathered on my list:

Question 1:
Do both mean the same or does it make a difference if I say 'Mi estas malpli granda ol vi.' or 'Mi estas pli malgranda ol vi.'?
I know from Spanish that you also can form the comparative in the reverse direction, something like 'less + adjecitve'. Is it the same here in Esperanto?

Question 2:
A negated yes/no-questions can be answered with 'yes' and 'no'.
'Didn't you see him?'
I could answer 'Yes, I did' as a contradiction and 'Yes, I did not.' as an affirmation.
So if I do so in Esperanto 'Ĉu vi ne vidis lin?' what does the answered 'Jes.' imply and how can I distinguish between a contradiction and an affirmation in Esperanto?

Question 3:
Reading some Esperanto, I noticed this verbal construction 'rigardas for'.
So I checked the vortaro and I found 'forblovi, fordoni, fordrinki, etc.'
My question is, is the infinitve of 'rigardas for' 'forrigardi' and if you conjugate the verb the preposition is split off of the verb and put behind it?
Is it like that: 'fordrinki' -> 'Li drinkas for.'?

Question 4:
How do you create conditional clauses in Esperanto?
I am not really sure how to create conditional clauses in Esperanto, so please correct me if those trys are wrong.

zero conditional:
If it rains, one gets wet.
Se pluvas, oni igas malseka.

first conditional:
If it rains this evening, we will go home.
Se pluvas hodiaŭ vespero, ni iros hejmen.

second conditional:
If I had much money, I would buy a new car.
Se mi havus multan da monon, mi aĉetus novan auton.

third conditional:
If you had called me yesterday, I would have come to your party.
Se vi estus telefoninta al me hieraŭ, mi estus veninta al via festo.

Are all of those conditonal clauses correct or do you have some correction suggestions?

Question 5:
I didn't find standard quotation marks in Esperanto's orthography but I read that they are based on European quotation marks.
Thinking about European ways for quotation, I found:
„quote“
"quote"
‚quote‘
‘quote’
'quote'
»quote«
«quote»
‹quote›

So which one should I use in Esperanto?

Question 6:
I don't know if it's just me but I often tend to pronounce certain 'n'-s like the 'n' as in 'ring' for example in words like blanka or lingvo, because it is much easier to pronounce it that way than to pronounce it like the 'n' in 'noze'.
Do I pronounce it wrong or is it usual to pronounce those Esperanto words as described above?

Again, thanks for your help, I really appreciate it a lot!

zero conditional:
If it rains, one gets wet.

Se pluvas, oni malsekiĝas.

first conditional:
If it rains this evening, we will go home.

Se pluvos ĉi-vespere, ni iros hejmen.

second conditional:
If I had a lot of money, I would buy a new car.

Se mi havus multe da mono, mi aĉetus novan auton.

third conditional:
If you had called me yesterday, I would have come to your party.

Se vi estus telefoninta al mi hieraŭ, mi estus veninta al via festo.

Note: you could just say 'telefonus' because the hieraŭ makes it clear that you are talking past conditional. But to just say 'venus' instead of 'estus veninta' could lead to a misunderstanding if there weren't sufficient context to make if clear that you were referring to a party that had happened and not one due to take place (perhaps that night or the following week).

Question 1:
Do both mean the same or does it make a difference if I say 'Mi estas malpli granda ol vi.' or 'Mi estas pli malgranda ol vi.'?
I know from Spanish that you also can form the comparative in the reverse direction, something like 'less + adjecitve'. Is it the same here in Esperanto?

Yes, it's the same way in Esperanto.

Question 2:
A negated yes/no-questions can be answered with 'yes' and 'no'.
'Didn't you see him?'
I could answer 'Yes, I did' as a contradiction and 'Yes, I did not.' as an affirmation.
So if I do so in Esperanto 'Ĉu vi ne vidis lin?' what does the answered 'Jes.' imply and how can I distinguish between a contradiction and an affirmation in Esperanto?

The "Jes." could mean either one by itself depending on the speaker's background. To be clear, people repeat the verb, so "Jes, mi vidis.", "Ne, mi vidis.", or "Ne, mi ne vidis." It's fine to ask for clarification. If you don't want to repeat the verb, then "Ĝuste" or "Prave" clearly mean "Jes, mi ne vidis." and the mal-words will mean the opposite.

Question 3:
Reading some Esperanto, I noticed this verbal construction 'rigardas for'.
So I checked the vortaro and I found 'forblovi, fordoni, fordrinki, etc.'
My question is, is the infinitve of 'rigardas for' 'forrigardi' and if you conjugate the verb the preposition is split off of the verb and put behind it?
Is it like that: 'fordrinki' -> 'Li drinkas for.'?

You can attach "for" to the beginning of the verb or leave it at the end, regardless if the verb is in the infinitive or indicative. The following sentences are fine:
Li manĝis la sandviĉon for.
Li formanĝis la sandviĉon.
Li volis manĝi la sandviĉon for.
Li volis formanĝi la sandviĉon.
Prepositions work similarly, but when the preposition is attached to the verb, the object of the preposition becomes a direct object needing the accusative.

Question 5:
I didn't find standard quotation marks in Esperanto's orthography but I read that they are based on European quotation marks.

I think I see "quote", 'quote', and «quote» most often, thought not necessarily in that order. Pick whatever's most convenient for you to type, I don't think a standard has formed though other Lernu! members are free to disagree. Don't lose sleep over quotation marks.

Question 6:
I don't know if it's just me but I often tend to pronounce certain 'n'-s like the 'n' as in 'ring' for example in words like blanka or lingvo, because it is much easier to pronounce it that way than to pronounce it like the 'n' in 'noze'.

Some consonant assimilation is OK as long people understand what you're saying. [saŋgo] is fine for "sango" but just [saŋo] could theoretically be confused for "sago".

Esperanto has no standard quotes. Feel free to use whatever comes naturally on your keyboard; that's what everyone else does, and Esperanto books are published with a wide variety of different styles, generally corresponding to the countries where they were published. No need to get fancy, everyone knows what you mean.

You probably read "European quotes" because the 'bracket' style of quotes used for languages like Chinese, Japanese, and Korean are basically never used with Esperanto (for no particular reason, though perhaps simply because Esperanto has a Latin alphabet, and those quotes aren't generally used with Latin alphabets)