Mixing/matching componentry for SS

I'm hoping to build a cheap single speed using an old mountain bike frame and wanted to use an old Campag Chorus rear mech I have with a mixture of things I also already possess or intend to buy:-

A Shimano UG51 chain, a Suntour 42t chainring, Shimano 300LX crank and 16t Shimano DX MX66 cog.

I'm assuming that because it's a simple set up it should work, but am I complacent to mix and match so many brands?

Mixing-and-matching parts from various manufacturers on a Single Speed is not uncommmon.

What you need to be aware of is that SOME BMX freewheels can use a standard ROAD chain like the one I think you are planning on using whereas other BMX freewheels have 'thicker' cogs similar to the size commonly used on TRACK cogs [3/32"(?)] ...

I don't know how thick the cogs are on the particular Shimano freewheel that you are considering ... it may be available in BOTH sizes, so ask before you buy.

WHY are you planning "to use an old Campag Chorus rear mech" on your bike? Does the frame have vertical dropouts?

I think that you will find that the STOPS on most rear derailleurs cannot be adjusted inward enough for use on a Single Speed to allow it to take up the chain slack ... in case that is what you want to use it for ...

... Of course, you might be able to find a long-enough bolt that you could substitute for the stop adjuster bolt.

A SURLY Singulator (or, equivalent) is the probably easiest way to take up chain slack when using a frame with vertical dropouts. In addition to the SURLY-type chain adjuster, there is a type which I will refer to as the PAUL-type (for want of a different label) which is bolted to the chainstay and the pulley wheel's holder is moved forward on the stay so the pulley wheel can remove the excess slack in the chain.