Answering the questions in the OP:
Ultra_Solo_Sig_0904 wrote:my worries are do they hold good enough to support your firearm and enough not to take the holster with gun if needing to draw?
Absolutely. I carry a full size Sig 2022, and the velcro clips hold very firmly to my belt. When I first put it on in the morning, I think it's going to go loose on me, but by the time I take it off it at the end of the day, I'm convinced that the holster and belt have become one. (I'm sure it happens much earlier than the end of the day, but that's usually my first check point.)
Ultra_Solo_Sig_0904 wrote:since i already have a gun belt so would i have to sew the velcro into my belt? or is the velcro have doubleback tape or something to stick onto the belt?
Like the other posters here have said, you can double-stick it to the belt. If you have more than one gunbelt, perhaps you can send one back at a time to the manufacturer to get velcro added?