Is this a good combination for Solas? Right now I don't have him doing much with Rift spells. What's the most efficient way to get the best use out of this tree?
The Rift tree is amazing. First of all Pull of the Abyss is one of the best CC in all the game, and all the Rift spells provoke weakness that, especially on hard or nightmare, is a Godsend as 30% less damage done by enemies is a lot. Plus with the Rift passives weakened enemies replenish mana for Solas, hence he can continue to use the various support spells on the party. A very good combo.
On the Spirit tree you need Barrier, Dispel (a must) and on higher difficulties also Revive is handy to have. Then you take Winter's Grasp, Fade Step (not necessary on normal difficulty but it is a good defensive skill and it is used by the AI only when attacked in melee) and Winter Stillness. Then you take the Rift line of spells (I usually put Barrier, Fade Step, Dispel and Pull of the Abyss as preferred choices). This is the core of the build and the rest is up to you. It is a build focused on support and Solas is the best at it given the Rift spec.
Dorian and Vivienne are better specced as damage mages (but Vivienne you can also make as a support, also if she will never be so good at it as Solas). If you go with 2 mages you can create very good synergies with the CC of the Rift spec and another build focused on nuking (something like Pull of the Abyss + Fire Mine + Walking Bomb, for example, is very powerful).