You have an entity ID conflict with another mod. Either delete the other mod, or you can maybe fix it by changing IDs in configs. Go to .minecraft/config/Galacticraft/core.conf and search for this line.
Change the number 170 to something else (less than 256) until it works.
Okay I found 10 identical ID's between 170 and 180 and changes core.conf ID's and mars.conf ID's.
Seems to launch now.
On a side question, does adding mods add lag ?
This is the last mod I decided to add but I'm not sure the kids will be able to go that deep.
Thanks for the info about OceanCraft, I'll mention that in future if anyone experiences a similar problem.
Great that you have it working OK.
Does adding mods add lag? Hard to answer!
Everything in the game adds some lag, 2 zombies makes more lag than 1 zombie, etc.
If you are running a server for let's say 10 players or less, and you have a reasonably fast CPU and no network issues, then in normal playing conditions lag shouldn't be a problem, like nothing anyone should notice - the server should normally be able to maintain 19-20 ticks per second (tps)
Some things inevitably produce lag - worldgen is one. So on a well managed server you will try to pre-generate all the parts of the overworld that players can easily reach, before the players come online. (It's a balance: pre-generating the world unnecessarily will make your server take more disk space than it needs to.)
Mobs and machines each make a small amount of lag, but you shouldn't notice it unless you have hundreds of them. For example, if a player made a base with 200 electrical generators and a complex item routing system with a lot of pipes, and items in the pipes all the time, that might make for some lag.
Anything in large numbers makes for lag - it could be a broken Buildcraft pipe on a quarry spewing hundreds of blocks into the world, it could be an out of control mob breeding program
Electrical systems make different amounts of lag depending on the mod and how it's programmed: in 1.6.4 and before, Universal Electricity (and Mekanism) were fairly bad lagwise for large electrical networks (e.g. 100 generators connected to 100 batteries via the same set of wires would make a whole server "feel" slow). Thermal Expansion was fairly bad, IC2 pretty good. We did a lot of work in Galacticraft in 1.6.4 to make it one of the good ones, and that has continued into 1.7.2 / 1.7.10. I can't comment on the 1.7.10 versions of other mods.
Sometimes programming bugs, or just bad programming style, in specific mods can make for bad lag, even crash a server, when certain things happen. ICBM mod is an example, its largest explosions will typically make a lag spike or bring down a server. In general I would hope that if server owners bring these issues to the attention of mod authors they will try to find a fix for it.
Galacticraft has a current lag issue in the Asteroids dimension only, we are working hard to find a fix. That's no reason not to set up a Galacticraft server now. But you should be willing to update the mod sometimes, to get the latest fixes. Sometimes updates can be server only, but if the version number changes (e.g. if GC 18.104.22.168 becomes GC 22.214.171.124) then the clients will need to update to 3.0.6.xxx as well.