#1
attention: terrible English!

I wanted to ask that thing, which i have no idea where it belongs, i mean which part of music theory to look into so here i am

I've learned the chords and the basics (major/minor scales) so i can play my fav. songs on guitar, that happened loooooong time ago. Now i wanna learn how to compose my own songs in my crappy home studio!

I ll give u an example cause i cant set a question right with my terrible english.

Lets take this basic chord progression G, Bm, Em, C.

My real prob is with bass and piano.

on bass ...

E string 3rd fret = G
A string 2nd fret = B
E string open = E or MAYBE A string 7th fret = E
A string 3rd fret = C

lets say i play the whole Verse with an open E to get a lower thicker sound?!? and on chorus i go up to 7th fret to get a higher sound .. its still the same chord progression right? but when i do that on bass .. do i have to do it on guitar chords and on piano too? if i keep playin that on guitar instead of that is this it it wrong?

----------------------------------0-------------------------------0
----------------------------------2-------------------------------7--
-----------------------that------2-------------------this-------9--
----------------------------------0-------------------------------9--

so now that i ve said my prob. i realized what i really want to know ..

when i go up an octave on bass .. do i have to do it on piano and guitar too? what if i wanna play the chord progression (on bass) really high .. i m still in the right key, but is there are any rules about that i should know? which obviously i dont know cause my knowledge on music theory is on the lowest level.

p.s Sorry for my millions of grammar mistakes.
#3
There are no rules. It's called theory for a reason. They're basically guidelines. If you like the way it sounds, go for it. Don't worry about the theory all the time, that will only block your creativity.