Mixing Formula brands?!?

Ok so my husband and I are expecting our 5th. We are a one income family so I signed up for WIC to help out with formula and baby food. In South Carolina they only provide Gerber Good start, that's fine, and it doesn't cover all the formula we will need. that's also fine, we've been buying it up a little at a time for the last month, yes I check the dates. I recently discovered that parent choice(walmart brand) makes a formula almost exactly like Gerber Good start gentle. my question is if it's pretty much identical is it ok to go between the two? I've switched my others before but stuck to one brand because they didn't have the tender formula before. any advice? any mommies use two brands? And the hospital we will deliver at uses similac, I forgot to ask if they provide Gerber if you ask for it but if they don't will it hurt his belly switching?