SCUBA BC Question?

Sounds like you can swap out wings and this is a harness. Nuthin' wrong with that at all. It's cheaper for you and means less gear to tote around, since your extra wing stows easier than a full BC. It's what I use. I can go between wings and switch between dual and single in minutes. I also don't have to re arrange how I rig, such that there's no removing or moving D rings or carbiners or any gear attached to the harness. The only issue is the gussets on the wings. Diving a single tank with lousy gussets can cause tank wrap on a larger lift wing, but a quality rig won't do that. Finally...you can never have too much lift. You never know when you'll come across a $100 anchor that's begging to come back to the surface. :) Is this an OMS or Dive Rite, by chance?

The easiest way to have one "BC" but have the option of different lift capacities is to go with a backplate and wing set-up.

The backplate is metal and with the harness you set it up to your body. To the backplate you can change the wing or the tank configuration quite easily.

With that said, the wing is usually the most expensive part of the BC. It may work out better to have two BCD. Both the wife and I have two sets of gear each. One set is for single tank recreational diving. The other sets are for technical diving.

I use a AP Valves Buddy Commando (old but bullet proof for UK diving).

It has loads of lift and copes with twin 12 litre steel cylinders which I use most of the time.

These are held on with simple "twinning bands" instead of hard bolted to a back plate. This gives the flexibility to swap to single cylinder if I need to.

As to your last question - you can't have too much lift (better too much than too little).