There are a few issues here. First, I don't think the Safariland holster will easily fit onto the belt, without a molle adapter. You MAY get in on there, with a UBL, but I think it will be a big hump. Now if you don't mind removing the "molle" webbing where the holster will attach, you might be able to pull this off, but you'd probably be better off with the Safariland molle adapter, even though this is gonna add some bulk as well. Another option would be to dremel some extra room in the UBL slots, to allow for the extra thickness of the molle webbing, but again I'm guessing their original intent was for you to use the molle adapter stuff. You might want to email them and get their take on things.
As to the Velcro only belt. This will (and has) worked fine.
But it needs to be cinched up "snug". So if you usually wear your trou looser and/or size up for IWB carry, you're gonna have to modify how you wear your pants. They should be fairly snug while standing. This style rig is not made for sitting around in all day. It will probably be a bit uncomfortable when seated. When the outer belt is mated, it will stay in place and not shift around, so the drawstroke is always there.