The Sinnoh set of Pokemon are available earliest.
Although Hoenn seems easiest, Ludicolo needs a Water stone which is available only after the HM Dive is obtained and the seventh gym defeated.
Similarly, the presence of Pawniard makes beating the 7th gym mandatory before proceeding to Route 9 in Unova.
On the other hand, in Route 217, most of the mentioned Pokemon or their pre-evolutions can be caught. This can be done before Gym 7 is taken down. Those not found in that route can be caught before.
However, if you happen to receive a Water Stone by trade, then it is possible to obtain all of the mentioned Hoenn Pokemon before Gym 4, as was pointed out.
