I think if the baby's head support is good, and they can sit in the Ergo without the infant insert thingy (so can straddle you), you can try the back carry, though it's kinda tricksy to get into. My DD is quite tall and had good head support, so I tried it once around 3-4 months. I wasn't super fond of it (still not) because I can't see her, and she won't go to sleep in the back carry like she will if we walk with her in the front carry, but it was fun for a change. But keep in mind...she won't be able to see over your shoulders so it's not quite as super-fun as you'd think a back carry would be.
For a long time my DD would only like the Ergo if we were going for a long walk (so she'd settle down and go to sleep after a while), and she'd crane herself backwards. Even now if we're walking, she'll flop one arm out so she can twist around and look. She was most unhappy in the Ergo (except when asleep) around 3-5 months, when she was too small to do the arm-out-and-twist-around trick.
My solution was to buy a ring sling and carry her on my hip for short jaunts. She was light enough that the weight didn't bother me very fast, and she got to see all around while still being snuggled up to me. We still do this for trips to the store, etc. but she's starting to get heavy (she's 9 months now) so I no longer carry her for super long stretches on my hip. :) But it's fine for 30 min, easily.
My sling is from sleeping baby productions, cost about $50 (we have the linen one) and has been an absolute lifesaver in lines and on planes, though we still use the Ergo the most! (Sling for jaunts, errands and around the house; ergo for walks!)