I'm wondering about the container size too. If this plant is basically the property of your cat, I would not want the container to have too much exposed potting mix which the cat begins to use as an alternate litter box. If so, then large pebbles over the exposed surface would be called for. If your plant goes from a south facing balcony a few feet inside the window also facing south, little change of its environment, unless there is a heating vent blowing on it. I've never kept one indoors in a container, but grasses are pretty hardy plants and even look good planted in a bird bath to simulate a water feature....which requires regular watering. However, mine planted that way died, so I'm no help telling you how to keep one alive!

Fountain Grass (Pennisetum) will frequently droop from rain, particularly heavy rain or overhead watering. It is especially noticed when the plants are in bloom. The impact and weight of the water pushes them down, and is exacerbated when the flower heads are soaked. It's hard to control the rain, but try bottom watering them (as in drip irrigation) and see if it makes a difference.

I can't help you on the PFG...no experience. I'd say it's already too late to plant warm season grasses in your zone. There isn't enough time between now and when they go dormant to get established. I'd wait until the soil warms up in the spring, at least into the 60's.

Well, I just planted 9 new pfg's to replace my winter dead clumps. May have had about 3 or 4 growing shoots on each clump, but it's just not worth it. Saw these on sale at the link below... fast growers, fast bloomers, won't take long to have those beautiful purple plumes again. :) I still envy the clumps down the road at Publix and Starbucks that survived this year...about 5 gallon size or so. I'll stick with replanting eah spring as necessary. I'd hate to take the chance on planting a 5 gallon grass only to lose it. Better off starting with the least expensive and watch how fast it'll fill in each year. Here is a link that might be useful: Purple Fountain Grass on sale