should i root
my phone ?
No you don't need to. Rooting is for adding additional function.
Most user won't find it useful.
is there any apps that is essential and it is a must to install
them ?
Yes there's a lot. For me, gReader, App organizor, TaskManager,
Handsent, OfficeSuite... and many more.
i heard the smartphone itself cannot play some sort of video in
some sort of formats. no flash too. any substitutes for these ?
For video, I would give up already. There's too many format on the
Internet. Samsung phone can do a auto-conversion when you copy,
that's why. But first ask yourself, do you really love to watch
movie on a 3.5 inch phone screen? About flash, can support in if
you intall certain app, but also not very useful personally.
can we use our own music/songs for ringtones ?
of course. it's a "smart" phone man!
can we use our own wallpaper instead of those pre-installed ?
Definitely.
how come samsung galaxy ace, i want set my pics as wallpaper/screen
saver, but cannot set full screen ? must crop out part of it ?why
must crop part of it out ? i dont want that. i want full pic. how
?
I think you can, as long as you keep the hight and width ratio. If
you are using G ACE, the resolution and the CPU(which is 600M
overclocked to 800MHz) is not so nice for watching HD video.