1: Is a 25G large enough for 1 or 2 of these shrimp, as well as 2 or 3 small, colorful fish?
YES
2: Do I need a sump for a tank that small, or can I get by without?
NO, BUT...you'll be glad you did if you can.
3: What kind of filter do I need? Is a normal sponge filter, or hang-on-back like...