Sheer number isn't really the issue, it's about balance and variety. Look at some of the past regional dex expansions. The Sinnoh Dex was expanded because some types were lacking in DP, to the point where there were only two Fire types (one of which was a starter!). The Unova Dex was expanded because people were complaining about being forced to use only new Pokemon in BW1. It's all about improving your options. Now looking at Hoenn's Dex, there's too main problems with the distribution. The first is that, like Sinnoh's, there's a lack of variety in types. There's only 2 Ice types (Walrein and Glalie), 4 Ghost types (Shedinja, Sableye, Banette, and Dusclops), 4 Dragon types (Altaria, Flygon, Kingdra, and Bagon), and 4 Fairy types (Gardevoir, Azumarill, Mawile, and Wigglytuff). Ideally, there should at least be 5-6 of each type, so all of these need more new families. Second, there's a lack of variety in the distribution. This mainly applies to the Water routes which are primarily just Tentacool and Wingull, but also other areas to a lesser degree. Some areas such as Scorched Slab and Rt. 123 are fairly pointless areas, and could use some good exclusive Pokemon to entice people to visit them. Take a look at my Dex idea
I agree that the game needs to have a wider variety of pokemon available... but there are certain limitations in terms of pre-E4 content. There's only one small ice-based location in the Shoal Cave. You can't just stuff it to the brim with Ice types. Similarly, the region where you'd expect Dragon types in most games (near the E4) is all water, so it's harder to fit dragon types into the set (also, you forgot Lati@s and Rayquaza on the list of Dragon types). There's certainly room for a few more Ghost types, though. Fairy types are a little harder to call, since the type didn't exist in the original, and there aren't that many to begin with - not expanding the Fairy lineup immediately seems fine, to me - let there be more post-E3.
With a little more pondering, I think I'd make a few changes to my suggestion. Replace Mantyke/Mantine with Frillish/Jellicent (that's +1 Ghost type) - indeed, Jellicent would be great in the water around Mt Pyre and perhaps on Route 130 (because it's near Mirage Island), and it's a line not available in X/Y. Misdreavus would be a nice addition to Mt Pyre itself, and could replace Meowth on my list. That would sufficiently build the Ghost type lineup, in my opinion. For Ice types, adding Seel/Dewgong should be sufficient (in addition to Lapras) - I'd replace Cherubi with it. Going through similarly to your list, here are the pokemon I'd add:
Skrelp: Underwater in Ocean [AS]
Clauncher: Underwater in Ocean [OR]
Alomomola: Route 128
Jellicent: Routes 122 and 130
Lapras: Route 125
Houndour: Route 112
Larvesta: Mt Chimney (added patch of grass)
Heatmor/Durant: Desert Underpass (replacing Whismur/Loudred)
Regigigas: Sealed Chamber or deep within Desert Underpass
Sunkern: Route 103
Hippopotas: Route 111 Desert
Shellos: Route 103 Surf (West Sea), Sootopolis City Surf (East Sea)
Deerling: Route 110?
Bouffalant: Safari Zone
Tynamo: New Mauville
Dewgong: Shoal Cave
Stantler: Safari Zone
Unown: Altering Cave
Porygon: Gift from Norman on return to home after beating his gym
Misdreavus: Mt Pyre
Ditto: Desert Underpass
Smeargle: Altering Cave
Kangaskhan: Safari Zone
Beyond that, I'd also move some pokemon around.
EDIT: On a related topic, what should happen with Altering Cave? I think it should get a couple of new pokemon in it, and then get pokemon showing up based on streetpass data.