Red/Black, huh?
Caveat: I haven't played competitively or bought cards since 6th Ed or so, so my info may be out of date.
I like the LotP with Fire Whip as a theme. I'd add some Fallen Angels or Sengir Vampires to add some lower (than the LotP) cost punch.
Use Unearth (1B sorc) and Phyrexian Reclamation (1B ench)- both bring back target creatures from the graveyard, Unearth requires the critter be 3M or less total cost, Reclamation requires an additional cost in mana and life.
To boost your mana curve, Dark Ritual was always the standard, but if that's not available, consider Burnt Offering (which gives you R AND B mana) and Priest of Gix.
If you can do that, choose your other red & black critters for their low cost (less than 3 total mana) and higher than normal impact, especially with effects that occur when the critter comes into or leaves play.
Goblins and Kobolds are fine from the Red side.
From black, consider Hidden Horror, Junun Efreet, Skittering Skirge, Spined Fluke, Dauthi Marauder, Phyrexian Ghoul, Phyrexian Broodlings, Priest of Gix, and Bone Shredder.
Bone Shredder can turn into a creature sweeper when summoned, sacrificed, then brought back via Unearth or Phyrexian Reclamation.
OTOH, you could use Mogg infestation in conjunction with any of this:
Mogg infestation
Destroy all creatures target player controls. For each creature put into a graveyard in this way, put two 1/1 red Goblin creature tokens into play under that player's control.
Used on your opponent, its a creature sweeper that clears the way for your bigger critters- esp. fliers. In combination with Breeding Pit or Kobolds, its a medium cost combo that instantly doubles the number of critters you control, makes them 1/1 and changes their type. Followed by something like a Phyrexian Ghoul, Phyrexian Broodling, or Fallen Angel, and they have PLENTY of food.
And for giggles? If you go that route, you might as well play Phyrexian Dreadnought, Renegade Warlord and/or Keldon Warlord.