>
> I start designing a new system (6 months for total development) and

I want to know if is possible use that version for testing?

>
> Because I dont have to do a hard work migrating from FB 1.5.3 to

2.0, later.

I suggest you start with FB 1.5.3 because you can not know when 2 will
be released, and it may leave you in an awful place where you are
forced to go live on a release candidate with possible serious flaws.
Although there are known bugs in 1.5.3 (see FB2 release notes), they
are more known.

There are very few problems with migrating from 1.5.x to 2.x, there
are a few ambiguous queries that 1.5.x will run that 2.x will refuse,
but providing you are not sloppy it is ok.

You should download and read the FB 2 release notes so you have an
understanding of the new version and can make sure in theory it will
work. Like any other software, you need to maintain it against the
latest stable build (1.5.3) and keep up to speed with the development
kits.

It is not like 1.5.3 will stop working when 2 is released, so you will
have some lead time to make any required changes.

Adam