hi guys, this may sound stupid. but are there any downsides to mixing new strings with old ones other than that it will require a new setup and the newer strings will sound brighter? I was screwing around with the tunning and had to replace the 4,5,6 strings with new ones.

I don't think it will, besides make you feel bad for wasting good strings the next time you change em all. It shouldn't really present any problems.
Gig Rig:

Schecter Hellraiser V-1
Crate BV120H
B-52 LS 4x12 cabinet
BBE Rackmount Sonic Max
Boss ME-50 Pedalboard
Digital Reference 2505 Wireless

I don't like BTBAM. Sue Me.


My Solo Project
If it was one or two I'd say go for it, but when you are replacing half of the strings might as well do all of them.
It’s really not a problem. Technically your older strings will stretch out and start sounding dull sooner, but if that doesn’t bother you and you aren’t playing live just don’t worry about it.