What do you do when a rush fails and/or you are halted in advance

You didn't yell at your teammates enough for lack of feed. (jk) 

You should build some scouts along with your fleet to kill the contruction frigates before they can build the strucures in both worlds.   Scouts are twice as fast as LRMs so they can reach the Con. Firgates earlier.  You should also try to use the Timed Explosive upgrade on the scouts to kill structures faster.

Its also important to place your fleet in a position to retreat when the battle goes awry.  If you're on the opposite side of the grav well from thw gravwell, you will lose a lot of ships during the retreat since your fleet is scattered and the other is concentrating on specific targets.  Losing 50% your ships before retreating is considered catastrophic especially during early game since you cannot build enough ships to replace them at that stage of the game.

When playing in a big team game like that, NEVER assume your opponent doesnt have enough resources for something. I played in the same situation with ReborN the other day except I was Vasari. I came in on his roid with 9 assailants and an egg, with some sentinels coming. I took down his constructors FIRST then took down his cap before it could get away. Unfortunately for me he got maximum planet health on the roid and had already built a temple of communion at his hw AND colonized an ice planet on the other side, where he was already building more labs. So in effect I slapped him down and pinned him, but I couldn't get in and attack his hw, not with the repair bays he had up and the illums he was pumping out. So I kept him pinned and waited for some help from my allies. It was all I could do, but it did help out my team. Later me and a teammate attacked him and got slaughtered by repulse and mass disorientation and meteor storms. In hindsight I realized i should have built an Orkulus there before engaging because I did have his fleet alone outmatched the whole game. I got a little cocky at the end I guess, and lost cuz of it.

My advice to you is to delay his construction of illums for as long as possible, destroy labs, constructors, whatever. Get flaks and then carriers after your first engagement and then jsut try and stay on top of him with tech. I had subverters with distortion field before he got guardians, and then started pumping out enforcers. As tec get those hoshis in ASAP and get Kodiaks super quick. Once my assault stalled on the roid I knew I had to keep my fleet larger and better, and it worked.

The problem with this is that level of tech - you could be researching something else hoshikos/carriers at that level

Yse, but if you caught the other guy off-guard, you won't get the carriers/hoshickos up fast enough to create enough havoc before the illuminator spam cometh.

With quick start he can have illums out in less then 5 minutes without having to colonise another planet other then his HW

Ill just give general tips on the importance of rushing...

Make sure your Shipyards are pinned . Nothing worse then forgetting to build a steady stream of ships. Happens to the best of us.

Anticipate your logistics. Cant stress this enough. Make sure your asteroid gets its logistic upgrade . Its so important that your transition from 1 - > 2 -> 3 military labs isnt delayed . Also you need the logistic to do double shipyard when you feel the time is right or you get additional feed.

Decide your planet bombing. Sometimes ive seen an EGG stuck bombing a planet for 2 minutes  , when it has a nano that could have been used in battle instead. Make your choice wisely . If you see an asteroid go from 1000/2500 health..then this factors in to your decision . You could lose a ship battle because your not using your abilities.

When a player makes muliplae shipyards , you can pretty much make the assumption that they are going to get fed. The most important thing in this case is to make sure your allies feed you.

Rally pointing. Theres nothing worse then seeing a battle where your outnumbered , and its because half your fleet is sitting on its arse at your shipyards. The worse noobs will say they are "building up fleet" ...urgh.

If your vasari , nothing makes an enemy shit themselves more then a colony frigate. Lol , as they all chase it.

How about the firepower of a fully ARMED and OPERATIONAL battle station???