if you're looking at friendly play then basically whatever you feel like using.... if you've got your eye on more competetive, tournament type gaming, then I would suggest these pointers for a newcomer to both guard and epic:
1. Activation count is very important, if you only have 5-6 very durable formations, you are often at a disadvantage to an opponent with 10-11 as they are able to out-activate you and then are able to act without you being able to respond, equally however, having 15+ activations in an army can often mean they are very fragile and unable to have much effect on the game, for Steel legion, I'd aim for 10-11 activations if possible
2. formation upgrades, while some are very useful (some form of ground based AA attachment like a hydra or hunter) most tend to beef your formations up at the expense of activation count, certainly at 3k, the game tends to favour having a decent number of activations rather than fewer, more expensive ones.... guard formations are already big and potentially cumbersome, adding extra models can often mean that they become less able to react to engagements and defend themselves effectively
with this in mind, I'd drop the hellhounds and demolisher upgrades, add a hydra to each company, then look at some other units, warhounds are excellent in guard armies, they are often able to speed around and neutralise threats with their macro shooting, they also synergise well with guard companies, triggering assaults and pulling in the support fire of an entire guard company can hurt almost anything pretty badly, some scouts would be very useful to screen vulnerable formations against teleporters, you can use sentinels for this, which are essentially a one-shot scout screen, with 6+ armour and being light vehicles, they are among the most vulnerable formations in the game, but they are cheap! also you will almost certainly want to include some artillery to enable you to hit anywhere on the table, and attack stuff trying to hide, manticores are probably the best choice here, although they are often a big fat target for enemy artillery and planes as well as other alpha strikes.... you may want to include some armour, either a tank company, or some superheavies, singleton shadowswords are just great fun, especially with a commissar to make them fearless and give them leader so they rally back from broken with no blast markers
I'd probably go with something like
Reg HQ + hydra
Mech inf + hydra
Infantry company + hydra
sentinels
sentinels
manticores
shadowsword
shadowsword
hydras/rough riders
thunderbolts
warhound
warhound
but that's just me, RichardL's all conquering guard list is similar, but takes a tank company in place of the foot infantry and shadowswords
best option is to try some stuff out and see what you like using
