Over time you'll be creating and testing 100's of portfolios.  Do you really want to keep track of 100's of Demo accounts?

Also, some brokers may place a limit on the number of Demo accounts you can create.  They may also place a time limit -- e.g. 1 month, 3 months -- and then the Demo account is deleted (along with all your historical data) -- which happened to me recently.

There are a few ways I can think of to limit the noise from other portfolios and strategies:
1. Choose your magic numbers wisely and then use Account History's sort facility.
2. Run a portfolio for a week and use Account History's 'Custom Period' to filter-out older strategies by specifying the date range a particularly portfolio was actively trading.
3. And there are probably scripts and indicators available that allow you to select a particular range of magic numbers for analysis.  Even if you are not a programmer -- this would be easy to code and could serve as a good first project.

And there may be other ways to roughly achieve what you are looking for.  Since you probably shouldn't be spending that much time in a Demo account -- after all, it's just a sandbox that only approximates real trading -- then it seems that creating a Demo account for each portfolio is way overkill.

I'm curious as to how others do their demoing and what broker they use.  I have been using LMFX as there are no FIFO rules but it's been a pain. 

Does everyone use 1 demo account and just add every EA/Portfolio to that?  I prefer to have 1 portfolio to 1 demo account so i can see how each is doing while demoing and I can have an accurate trade history and balance with no noise from other portfolios in it.  I am finding it's a little annoying to run multiple copies of the LMFX MT4 open at once but it works for what it does.

Thanks
Dan