Canna is a very vigorous grower that does not require fertilizer in normal garden soil. The pot size is an issue mainly to keep the plant from falling over as it puts on more height. Al

IMO, vermiculite is a less than stellar choice for starting seeds. It collapses/compacts very quickly when top-watered with anything other than mist, causing it to hold too much water and remain saturated, robbing seeds of much needed aeration - and it's pH covers a wide range (from about 6.0-9.0), but it is usually mildly alkaline. Seeds will appreciate something that is sterile (to prevent damping off fungal problems) and reliably holds lots of air. Sphagnum peat, because of it's low pH is, practically speaking, sterile. Peat mixed with either sterile Turface or calcined DE (NAPA Floor-Dry) @ 1 part peat:5 parts Turface or DE is a great seed starting mix - and it will not collapse. Fill cell packs with this mix and you can allow seedlings to establish to the point that transplanting is a breeze. Al

Feeding seedlings is optional. Some do, many don't. Feeding or not depends on the growing mix you are using and how long before they can go to the garden. Looks like 3 there. Those 3 need to be separated and two of them repotted. Bury most of the stems too. Main thing is if you do feed them be sure to use a well diluted and a nutrient balanced mix and keep it to a minimum. Root development, not top growth, is what is important at this stage. Dave

Don't know what "bumper crop" is but if your soil is already well amended with nutrients and micro-nutrients and has a balanced pH then additional feeding shouldn't be needed until after first fruit set. However many do work some Tomato Tone or similar fertilizer into the planting hole soil at planting time and some have their "swear by" hole prep recipe of many ingredients. It's one of those optional "can't hurt, might help" things. "Hole prep" (what feeding at planting is often called) is a your choice option. It depends on too many variables for anyone to give you a specific answer - what the plants have already been fed if anything, what they look like in terms of color, how old they are, what all is already in your soil, how deep you plant them, etc.etc. If you search 'hole prep' here you'll find many different thoughts on the question. Dave