I'd go for if i were you;
6 x Tiger barbs
2 x Kribensis
1 x Bristlenose pleco
You need a plec that will stay nice and small for your tank, around 5/6 inches and the BN fits this nicely. It will also keep the tank clean. You will need to throw some driftwood in for it though, but this will only help with your aquascape
HOWEVER, I cant say getting a pleco for the sole purpose of cleaning your tank is a good idea. Get the fish because you love it, not for a job. Having a BN/any pleco will also not change at all your cleaning/partial water change schedule btw, whilst it would help with the algae plecos are constantly graving and therefore constantly pooping....... so IMO its 6 of 1, half a dozen of the other...
God knows there has been a lot of discussion on this one lately that i've seen - sand/gravel.
I now have both. Go for sand. Your worried about clean - well with gravel it sinks through, gets messy and needs a good hoovering every so often to keep it clean. With sand it sits on top of and is much easier to just suck off rather than having to 'dig in.' Sand is a lot more work putting it in - needs rigorous cleaning - but well worth it long term i'd say. Especially with you wanting to feed guppys - you dont want rotting fish sinking into your gravel. A BN pleco will also probably prefer the sand.
Tiger barbs are an excellent jumper and so id say get a hood!
The 150w will be fine for a heater.
So will the 50 gallon filter.
Kribs breed in caves so if you want this, well, you'd need some caves. haha.
I've just back-read a lil more too and stil no water params? Get them - because if you plan your tank and it doesnt suit your water at all, thats going to suck for you and your fish!