Then you're stuck with the problem of what they would do with the electricity, when they'd got it. Lighting? Radio? What is essential when you already have working magic?
That's where the fly in the ointment comes in, I think. Making the world consistent on this point. It's like ERB having a society where they use swords -- and radium pistols. I strongly suspect that, for all his storytelling abilities, were he to try that today (rather than in 1912), it wouldn't get off the ground.
Mixing magic and technology -- especially advanced technology -- is a very difficult thing to pull off because of that: if you have magic, what's the point of technology. The electricity idea you describe above being a very good example. Why not simply have a spell which substitutes, rather than taking the charge out of, etc.? A spell to power (whether to provide light, energy, what-have-you) a locality, rather than half-measures, where neither is really satisfactory. Keep in mind that, when writing such fiction, the magic must also obey laws -- it must become a kind of science in itself, as it were; otherwise it simply becomes a constant
deus ex machina, and that can simply sap any tension from the story.
Incidentally, there are also other problems with giants, aside from the food situation. It seems inherent that giantism puts a severe strain on the organic system, and giants tend to not only have horrendous health problems, but very short lifespans. This wouldn't be all that efficient economicall for your society, unless there's a way around that. Magic might accomplish it; but breeding programs (even with magic involved) would likely
not do so.
You can overcome these problems, but you've got to take them into account and compensate for them -- and that can eat up a lot of space in the text, unless you're very careful. In the end, if you choose to mix these, it is going to require considerable planning ahead; it can't be slapdash, or it will come down like a house of cards (there I go with the trite similes again... gak!). Unfortunately, that's something where people aren't going to be likely to help a great deal without knowing an awful lot of detail (including, probably, a sample of the writing showing how you handle this sort of thing)... so the best rule is simply to remember that, if magic can make it work without the technology, or vice versa, it isn't (generally speaking) a good idea to mix the two....