External Preamp and compressor for my Focusrite 2i2 interface !

If you like your sound no need to add anything.  And quality external preamp will probably have more gain (10-15 dB) which can help w/ mic placement and certain mics and singers.  It could have a nice transformer in the in or out or both, providing a nice round tone instead of a sharper-edged interface preamp tone.  this is what many people associate w/ tube gear, when what they are hearing is old tube gear w/ big transformers.  There are lots of reasons for a high quality preamp, but if you are happy w/ your sound you don't need to be in a hurry to plop down $400 unless you have the money to spare and have no greater need (as LPMike points out, a good mic should be first on the list).  The difference in sound is one of degree, not kind, and the standard interface preamp can capture good sound.

Same w/ a comp.  A few dB of reduction going in can smooth out a bouncy performance which means you don't have to slam your virtual compressors so much during mixdown.  A good compressor too w/ help add the sonic signature of high quality components we associate w/ professional recordings.  But again it isn't necessary - good recordings can be made w/o a compressor.  They do make life a little simpler while recording, tho.

If you are new to recording, it will take time to learn what to listen for and how that translates over different systems.  With a good mic(s) that you spend a couple of hundred on and a modern interface w/ built-in preamps you can go a long way and learn what you can until you think you need a bump in hardware.

If you have some money, my recommendation would be willing to spend enough so you don't keep buying the same thing over and over again.  Preamps - a Rane and the M-Audio DMP are the cheapest that are better than what the scarlet offers, and they are about $200 (the DMP is stereo).  Up from that is the Warm Audio, Focusrite ISA One, Gap 73 or FMR RNP, from about $400 up.  Those are ones you can keep and use even when you have gold albums on your studio walls.
 
I don't think a stand alone compressor will do you any good, since I doubt if your scarlett has inserts on the preamps.  ART makes a channel strip for about $350 which is highly thought off.  Preamp, compressor and EQ, with a tube saturation element.

The best thing you can do is buy a quilt and hang it in the corner of the room you are using to record in.  Face into the room when you sing or play.  Most of the room sound the mic will pick up comes from reflections from the wall behind you and a nice quilt will dampen that sound considerably.  You'll have less reflection sound with the attendent peaks and more dry sound.

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