just use whatever you have to shoot the ROM for them, but you need a hotshoe flash, your friend don't ever bother to fork out $200~$300 to hire someone, why you need to buy a flash to use on them? if you think you can buy and use the flash in future, by all mean.
many couples think ROM is a very small event, not willing to spend money to record it, not that they can't afford, many just think is not necessary, so this is really up to them.
I have seen many couples just happily let their friends or relatives using PnS to snap away during ceremonies, out of 10 couples, maybe only one or two couple really hire a photographers. this is the reality.
the ceremony itself is very short, less then 5mins, you have to work very fast, get your setting right before you go inside the room, (presume the ROM is take place at the Registry Office at the ground floor of ROM building). just stand next to the registrar, you able to get most of the shots from that angle, bear in mind that the room is very small, so don't think that you are able frame everyone in your picture, just concentrate on the couple, remember, ceremony is very short, no time for you to review your shots, and you miss any of the shots, that's it. don't forget to get a group shot of the registrar, the couple and the two witnesses, before leave the room.
when outside the room, can go and take all the group photos, when you are in doubt, just see what people do, than you follow.
ps, just notice you mention the ROM is take place at SRC (Singapore Recreation Club), well, can spend money on the venue, reluctant to hire a photographer, that's the reality.
good luck!
