Yes, it's OK to mount HDD's in adjacent slots. However, it makes sense that separating them by one empty slot may give marginally better cooling.
But there's another point I think. You seem to want to do this just so that you can remove one HDD rack, leaving the airflow path from that front fan unimpeded. What you're not considering is this: the reason the HDD cages are mounted right after the front fans is so that the fan air flow will cool the HDD's very well before the air travels further into the case. The case is designed for some airflow interference by the HDD's. So changing it as you suggest will give you slightly improved case ventilation but may cost you reduced cooling of the HDD's. Consider that trade-off in making your choice.