Do you mean weaning on to formula milk from the breast?
It's fine to mix feed or move to formula feeding if you want to. Or stay breastfeeding. The answer to your sleep issue will probably come, in part, from needing to feed more frequently in the daytime.
Don't give your baby non-milk solids, that would be completely ridiculous and your hv won't have advised this just for dropping a centile.
HV don't normally get concerned unless two centile lines are dropped. Moving above or below one centile line is not unusual and no significant cause for concern, except noting baby may want more milk so offering more.
It's perfectly reasonable that a baby needing lots of calories will have a full feed every 2 hours during the day. If the feeds are smaller amounts they may be wanted even more frequently.
As well as offering more milk in the daytime (and by daytime I mean 7am-11pm, your awake hours in the day), which should be the primary focus, I would also look at settling methods.
Have you developed a way to get baby to sleep that is not feeding? I found a dummy and a hand on baby's chest whilst in the cot worked best.it is quite possible that a number of these wakes in the night are baby just wanting help to get into a deep sleep, rather than hunger.