Newbie - Various questions

So, I have learned that my soil is mostly clay. With some Ground Elder.

So I'll be using containers for everything, at least this year.

The container that I have in large quantities (and can get a few more for free) is a white, plastic bucket, that measures 24 cm in diameter and 22 cm in height.
Right now I have 3 of them with a strawberry plant in each and other 3 with 3 or 4 lettuces each (cut and come again).

I have a few leeks that I started from seed, and will also buy a few seedlings. I started the seeds a bit to late and still have another 5 weeks before I can transfer them.
Can these buckets hold more than one leek? I was hoping to get 3 leeks per bucket.
I have some bottles to cut up, to put around each leek, so I'll have more height for blanching.
Also, our last frost date is around May 15th - 20th. When can I put the leek and pepper seedlings in my greenhouse? It's not heated, but it gets a lot of sun the entire day, so it gets quite warm.

I will be buying a couple of tomato plants soon, and intend to use a bucket for each of them. Or are this buckets to small?
I can also stack to tires on top of each other, which will give about 35 cm in height and 30/50 cm in diameter.


And now on the soil.
I have some mulch to put around the plants. If next year I mix it in the clay soil, together with some compost. Will it be enough for potatoes and leeks? Maybe even some broccoli.
For next year I might also raise the soil another 5-10 cm with good soil that would go on top of the clay/mulch/compost mix. But that depends on how much it will cost in sales.
Thank you.

 

I was really hoping to be able to get more than one leek per bucket, mostly because I don't have much space. I'll start cleaning the tires, and hardening the leeks. I actually hadn't thought about the tires for the leeks, just for the tomatoes, but it will work too, thanks.

I actually was questioning if the mulch would add enough drainage to the clay soil. As for compost, I don't have much. If I can rebuild the one or neighbor tore apart, I might be able to get some free compost for next year. But for now, it's nearly all destroyed or filled with ground elder.