Good questions, Martiansky. You are “thinking right”.
From my point of view…. There is practically no limit to what you can assemble with parts from a myriad of sources, relatively simple tools, and a few tons of ingenuity. I’ve been cobbling together pedal-powered vehicles for a dozen years, and sometimes I amaze myself at what I accomplish – yet I know I am only scratching the surface of what is possible.
I hesitate to even try to contribute to your specific questions, since there are countless ways to approach every detail. There is simply no substitute for experience. I keep dismantling Stuff that I have previously been afraid of, and discovering new wonders that I can make good use of in some way wholly unintended by the manufacturer.
Blah, blah, blah.
Bicycle wheels are not well suited to tricycles. Bicycle wheels are designed to carry loads only in direct line with the wheel, never sideways. When you ride a bicycle around a corner the weight remains directly up-and-down on the wheels because you lean the bicycle to compensate for centrifugal force. And bicycle wheels are made to be light-weight.
So bicycle wheels subjected to sideways force often turn into “tacos”. Or at least potato chips. That is, they suddenly warp beyond all repair.
Better to use sturdier wheels such as garden cart wheels, just as an example. These resemble bicycle wheels but are sturdier, with thicker spokes that are welded in place. They have bicycle-size tires, and they can also be found with solid rubber tires to eliminate punctures.
Motorcycle wheels are fabulous for a pedal-trike. Although they are also engineered for straight-up-and-down loads, they are massively strong for our purpose.
It just dawned on me that spindle-mount front wheels used on dragsters and dune-buggies would be awesome Playa-trike wheels. I’ve seen used ones on eBay. It’s all about brainstorming and discovering.
You say you want to utilize a bicycle frame you already have? That’s how I built this little oddity:Thanks to Mary Stevens for the photo!
There are still parts of the original frame present. Everything else grew from that like a runaway case of cancerous elephantitis. A key part of the rear axle assembly is a remnant of plain old black pipe from the plumbing aisle at any hardware store. The rear frame members are an old chain-link fence gate.
As for bicycle wheels you already have, some Kinetic Sculpture Racers assemble wide and sturdy wheels by stacking several bicycle wheels together and running a home-made axle thru them all. Bearings must be adapted, but that’s perfectly doable – I would perhaps first look at wheel-barrow bearings.
To stack bicycle wheels, since the hubs are wider than the rims, you use a ratio of complete wheel – rim only – complete wheel. And you tie the rims together with straps of flat stock or whatnot.
All this is great fun, but now I must go to an appointment.