Anyone can attend any Public Cosplay or Related Events/Conventions.
Even for those that have Entry Fees for the Main Convention Area like AFA, some times there're places close to it that dun need any Entry Fees to hang out at (like the recent STGCC where Hall A was "provided" for the Cosplayers).
You can wear your Full or Part of your Costume to the Event since this is Singapore (Only Country you cannot do this is Japan) as long as it's not Military Looking or "Indecent Looking". For the Military Outfit, depending on the design, most people just wear the basic Jumpsuit to the Event and put on the other parts at the Event Area.
BUT, based on what happened at an Event earlier this year at Funan (and a few years ago for Mascot Parade but that involved some Jewish Tourist), try to avoid Military-like Outfit for Events at Public Open Places; I heard some "Aunty" or "Uncle" called the Police when she/he saw SWAT Cosplayers at the Funan Event!
Weapons, especially those that look realistic or like Real World ones (Guns, Rifles, Swords etc) are highly recommended to be kept in their Bag or wrap up, covered from the Public View till you are at the Event Area.
And be civilize in handling your Weapon as there're still public at the Event Area; no one want a sore eye or shoulder from getting hit by your extra-large weapon!
There are Toilets at Suntec City; a few Cubicles outside the Convention Halls and at the Shopping Centre side of the place, and Whole Rows of them inside the Convention Halls.
Or if you already "decently dressed", can also change at any corner of the place, away from most public view (aka NOT outside the Convention Halls Entrance/Exit or along walkways where lots of people will be walking pass almost non-stop).
Yes, you can cosplay any Character you want tho' careful care in choice of costume is recommended to be taken if your size are on the extreme ends of the Size Bandwagon.Then again, this 99.9% of the time only applies to Females. >_<
Yes, you can Solo Cosplay; dun need to find Cosplaying Friends or in Groups.
Being in Groups is good in "attracting" more attention and allow your belongings to be taken care by fellow teammates or by your Minders. And if you are interested in joining an existing Group, can check the Cosplay Grouping thread or posting at Singapore Cosplayers FB Groups at Facebook.