If I have a hero who lost most of their help in battle, how do I get more help from town? The area where I got Pixies before now says none are available? Thanks

All creature dwellings (both in towns and on the map) produce a certain number of creatures each week. If you've hired all the creatures available in a dwelling them you'll need to wait until the next week before more creatures are available from the dwelling.

Thanks again. One more - how do I get new heroes to learn spells - as in, the heroes that I recruit after the game begins? (not the starting one)

I don't remember whether they learn the spells automatically by visiting the town with magic guild, or you have to open the magic guild screen. In Heroes 2 you have to open the MG screen.
And of course, the hero must have a magic book. Magic heroes (wizard, necromancer, battle mage, etc.) have them by default, might heroes (knight, ranger, etc.) have to buy them first for 500 gold by clicking on the magic guild in town.