For starters, fat-X's are different and have different potential food hauls and have different capacities to support specialists. Whether running a specialist-based economy or a cottage-based economy (or as often recommended, a hybrid), you will also need production-focussed cities to build your military. Later in the game, with State Property, Replacable Parts, Electricity, etc, tile improvements' respective

,

, and

contributions change - and as such a city's focus and potential is dynamic.
So therefore, before anyone gets to wound up about
always sticking cottages on floodplains, it may be well worth taking three seconds to evaluate the food haul of a city site.
I would also consider chain-irrigation prospects. Say your site is along a short river that's the only fresh water source leading into an area consisting largely of Plains, your inland cities' growth will quickly stagnate unless there's a pathway of fresh water, and irrigating a Floodplain may be your only opportunity to eventually bring adequate food to a Plains-heavy region.
So, with that said...
1: Why is it that lots of people say that floodplains are best for cottage spamming are not they equal good for a specialist economy citys both cities needs lots of food?
...'people' like cottaging Floodplains because they are better than self-supporting but indeed still add a

to the city food haul, and furthermore quickly burgeon into commerce-rich Towns. You get an extra

by irrigating a Floodplain, and that essentially carries one free specialist in a specialist-based economy. If you have the right Civics, Wonders, and other factors that add to the value of specialists, and their output is in line with your broader game objectives - then great! Alternatively, that farmed Floodplain may be supporting a Gold Mine that doesn't require a lot of turns to generate bags of commerce like developing a Town does.
2:In an se economy how big should i let the city grow before starting yo put in specialists in it if i play on noble i can wait a while becuse the happy cap is higher but will i not come after in research then?
There is no right or wrong answer to this one. It depends upon the development of other aspects of your empire, your rivals, your capacity to defend yourself, your objectives in terms of any likely war targets, your access to resources, leader traits, and so forth.
Personally if pursuing the specialist-based economy precept, I play a 'regulation' game until I have Literature and therefore access to the National Epic so I can better leverage the

points from the specialists - but I also use the whip a fair bit, so farming and food-rich sites suit my gamestyle anyway. Ultimately there are too many competing variables even to take this advice 'too much to heart'.
So in a nutshell - consider the food haul of your cities, consider the chain-irrigation needs of surrounding terrain, and consider the balance and directions of your empire. I know that this is a 'fuzzy wuzzy' answer, but it's a 'fuzzy wuzzy' game for something that's so mathematically founded!
