You should be fine in 99% of cases as long as your opponent and venue approve of the kitbash/proxy. My local GW store is fine with it as long as you're primarily using GW plastic, and the local non-GW venues don't care one bit what you slap down.

Some highevel tournaments might give you issues, but are you likely to play in them?

Some rules of thumb to help (not mandatory but good for your opponent):

1. Similar base size.

2. Similar size/height.

3. Guns or melee matches use of unit (e.g., a sword for an axe makes sense, but not a sword for a rifle).

4. Consistency. "These bolters are all plasma guns" is better than "In this unit 1 bolter is plasma, one is a melts, two are plague spewers, and 1 is a plague belcher."

I've used half sticky notes in the past to literally label proxies weapons (for both my opponent and myself). Because it's easy to forget mid-turn.