If you can't buy a certain evolution then can I just add cool parts into my eidolon's structure. He's a serpentine with the skill intimidate. So when he uses that can I give him a frill(that will pop out with his forked tongue) and a rattle. They will have no other use but to add in some cool effects. Another thing I wanted to do is when he speaks(which he won't do often but he has bluff and I don't so...) I want him to draw out the s's like a hiss, can he also do that. Am I allowed to do these things or is it the DM's call? Thanks:)
your eidolon appears as a fantastic creature. So long as your within the game mechanics he can look however you want.
But would the frill, rattle, hiss and tongue help with intimidate? Would it give it a bonus or no they're just for decoration?
Only if you take the Skilled (Intimidate) Evolution. Then I'd say those frills/rattle/hiss/tongue would be worth a good +8 racial bonus to intimidate :P
...though a DM is totally fine giving small bonuses/penalties to it based on looks...if you describe it as all cutesy...then I'd give you negs for intimidate...:)
Now now, cute doesn't mean you can't be terrifying. I invoke TVtropes