For tomatillo just remember that you need at least 2 plants to pollinate each other. Plant them about 3-4 ft apart but no further apart otherwise you lose the cross pollination effect. They can grow up to a metre wide so do be aware of that.
The main thing is that you have water and enough of it to last all summer. So set up a guttering and water butt system as soon as you can.
For concrete floors, whatever you grow needs really good deep beds otherwise there isn't enough soil to sustain if grown in too small pots. I recommend getting hold of some pallet collars and have them 2 deep and fill with topsoil as compost alone isn't dense enough IMHO. For my little 6x8 greenhouse I bought a few of the small ones from B&Q to fit round the door and used the normal pallet sized ones bought off ebay for my main growing space. In fact I brought many of them to France with me. They were a game changer in greenhouse growing.
As for growing, yes to the excitement of growing all the things but don't waste space on stuff you won't eat, or stuff that grows perfectly well outside. Achocha, cucamelons and squashes for example should be fine outside in the midlands of the UK but take up loads of space in a greenhouse. Peppers take absolutely aeons to ripen and you get one or two meals from them whilst chillis ripen over a much longer period and you get loads more meals out of them.
Use the frame itself to grow vine tomatoes up twine, and in between them grow either bush tomatoes, dwarf ones or pop your chillis in there.
Fill any gaps with basil and grow more than one variety and then chop it back when it gets bushy and freeze your chopped back stuff and discover the wonders of basil tea.
If you are going to grow cucumbers or melons, again use the frame to grow them up and keep them together for pollination purposes, try growing smaller cucumbers and keep picking them to keep them producing. Cukes are ready from a few inches long, you don't have to wait to pick and eat them. Prepare yourself for the melons to ripen overnight and then be eaten from the inside by insects, so the moment you can smell them, harvest them. Or you will lose them. And grow smaller ones not huge watermelons designed for the southern USA states.
For now I'd suggest planting some early potatoes in large pots of rich compost for a harvest in 12 weeks. I'd sow a few pots of carrots in sandy light compost in a deep pot, and I'd get a growbag and pop it on a tray, make some crosses [probably about 30 in a grid] and sow a pinch of lettuce and spring onions and beetroot in each cross. When I'd got the deep beds all in, and the lettuce and beets and onions were germinated and growing I'd transplant each bunch into the main growing beds which will otherwise be plant free until the tomatoes go in, and it would give me some harvests before then. Any remaining by May/June can be replanted again outside by that point.
And lastly, when you put your tender plants in, don't forget that even if it is hot in there in the day, you can still get very cold nights so make sure you stick to the weather apps and if a frost is predicted, get in there last afternoon and cover your plants with fleece or netting. Even after hardening off.