Posted: 24/03/2015 at 04:57
1 25cm pots are fine, plastic not clay. Long tom pots are deeper. Growbags don't really have enough depth so most people who use growbags sink a bottomless pot on top of the growbag. I use cardboard carpet roll because there's a carpet fitter round the corner and I asked for permission to take from his permanent walk-in skip. I'll probably go with pots this year as I cleared the co-ops of their planters at the end of season (1/4 price or so)
2 I usually make my own* feeds if using pots but the own brand is usually as good as the proprietary (though compare the NPK, micronutrients and trace elements. * (wood ash, banana skin, magnesium sulphate, garden compost, crushed bones, urea, seaweed etc).
Generally speaking, to support the green growth one gives the plants more Nitrogen (N), when fruiting something with more Potassium (K), though still use a broad fertiliser. If one is too selective such a narrow spectrum approach throws other nutrient uptake out of kilter eg too much K the plants won't take up Mg. If you use a decent growing medium you will only need to tweak nutrients at fruiting. It's worth reading up on plant nutrition.
3 Perhaps you could plant direct into the soil inside the greenhouse? The plants would get a good root run, you wouldn't have to be too particular about watering which frees you up if you need to go away for a few days. Composts vary widely even in the same product and certainly year-to-year.
4 I've never got round to co-planting toms and the Tagetes (marigolds) I've grown from seed. I've always usually short of pots by the time the marigolds are ready for potting on or the toms are ready for the final potting up.