Also, probably a stupid question, but for IWB use does a guy go a size up in waist?
Depending on the pants, gun size, and holster setup, you can expect to go anywhere from 2-4" up in waist size to comfortably fit an IWB setup. A lot depends on you, your needs, the conditions you're expecting to be in, the shirt/jacket you prefer or need to wear, etc. You want to try for consistency in where that gun sits, but it is also very much not an absolutist thing.
The bottom line is to be flexible in your mindset and just do what you need to do (often through experimentation) to find out what works best for you in your most likely situation.
Also, it's a process - most of us have a drawer or bin full of holsters we've tried, or use in certain specific situations.
There's a big advantage to having a common gun, common optic, and common light when you're hunting for holsters. My EDC has both a red-dot and a light - but it's an uncommon model (DWX Compact) and there weren't a lot of holster options for it, let alone with a dot and a light. But I found a number of excellent options at LegacyFirearmsCo. For less common gun/light/optic setups, you have to hunt around for a holster maker who covers that model - they need a mold-gun, and in my case it meant they also needed the exact optic and exact light on-hand to mold the holster around it all. So, not all shops, especially smaller ones, will have everything on-hand to do your specific setup. Don't expect any given maker to have any given combo - some will have a bunch, others will have a different bunch, and you just have to dig around.
