Sorry for the delay, bro. ;_;
The typing of your team is relatively good, but you're lacking ANY Ice resists... That is a huge problem. You aren't going to see any hail teams to often, but Ice Beams are being thrown everywhere, and plenty of anti-dragon pokes will be throwing HP Ice and other various moves too. You probably won't need to be worried 'bout STAB and boosted Ice move, but you could really use an Ice resist to lessen incoming Ice moves aimed at Latios. A random Mamoswine would love to throw Banded shards/fangs at your team. So yeah, one Ice resist would be nice.
Your fighting weakness is covered even without Cresselia, but be warned; an enemy Terrakion you can't stop may just CC it's way through your team. Gyara lacks intimidate and Latios is not bulky at all, so that could very well happen, especially if they get 'Rocks up. You need a wall that can stop boosting fighting types, which is why I was a tad skeptical 'bout Skarmory. Another fighting resist/immunity which is a bulky Pokemon would be a great anchor to the team. A Ghost type would fit nicely, since Dark and Ghost attacks aren't horribly common as is, and you have them covered anywho. Not to mention your Ghost might be able to block the occasional rapid spin; and teams that do rapid spin tend to be weak to both of the hazards you want up, which could easily be the difference between a win and a lose. Of course, only defensive Ghosts can spin block since spinners will carry a hard-hitting move(s) so ghosts that sweep don't ruin their day, so it's up to you if you want more sweeping power or a bulkier team with your Ghost of choice.
As for synergy, I'm not seeing an incredible amount of it aside from type cores. But what synergy is there is very important. Your hazards let Latios, Gyara, and Terrakion finish up the opponent after one or two of them tore some nasty holes open in their defenses. You only have one stat-up sweeper, but are more than capable of pulling a sweep out of your hat since your offenses aren't lacking.
And while you aren't running a full offensive team, your entry hazards make up for it quite well, meaning you have deadly sweeping potential without sacrificing defensive bulk.
However, you do have a single somewhat prominent weakness: your team is not slow what with Gyara (after a DD), Latios, and Terrakion, but it lacks at least one Pokemon with dangerous speed. The Pokemon in the last slot needs to have a Scarf/very high speed to alleviate this deficit, or very bulky so that incredibly fast Pokemon aren't such a threat. I'd aim for the later in all honesty, since having 5 attackers and 1 odd defender is the middle ground between a balanced team and hyper offense team; and in this case, the middle ground is not good. A 4-2 ratio is the definitive of a balanced team, and I'd think this team would do quite well as being balanced.
But yeah, patch up this "weakness" with your last Pokemon without opening up any others, and you've got a great team I'd say.