Question :

2 offices are next door to each other, with their own subnets & have a LAN cable ready to connect both of the networks together.

Office A is using 10.0.0.x subnet with Internet gateway on 10.0.0.1
Office B is using 192.168.1.x subnet with another Internet gateway on 192.168.1.1

I need to join these 2 subnets together so that both offices can see each other's PCs, network printers & file servers, but must not use the Internet gateway that don't belong to their own subnet.

Is the RB250GS is able to achieve this or I need to use the RB750GL?
(These 2 hardware are the only choice available for purchase off-the-shelf from the store I frequent)

Perhaps you can bridge or switch networks together. Then do src-address filtering on gateways to drop traffic from one of the offices.

I'm sorry, but that's not very clear since I'm very new to Mikrotik products.

Do I purchase the RB250GS switch and the SwOs is able to do what I required?
Or I purchase the RB750GL instead & the RouterOS in this unit is sufficient to do what I require?

You need a router, so the rb750.

Simply set the rb750 with both ip address using different eth ports.

However, it really needs to be the default gateway otherwise clients will not know how to route across the networks. You will need to set a route manually on each pc

Surely you are not serious about setting a route manually on each PC?
If I have 200 PCs on each subnet, that will take the whole day to do it.

Without setting the RB750GL as the default gateway, I could just :

route add 192.168.0.0/24 gw 10.0.0.x (ether1) on Office A's Internet gateway
route add 10.0.0.0/24 gw 192.168.0.x (ether2) on Office B's Internet gateway.

Wouldn't that resolve the problem in within a minute?

That would work assuming your default gatewYs support it - i made an assumption you couldnt touch them. Edit: youll need to set them to forward to the rb ip address on the same subnet as oposed to a remote subnet.