The AFG31000s has an accuracy of ± (1% of setting +1 mVpp), so a 50mV signal would have roughly ±1.5mV of noise when producing a 50mVpp. The instrument’s input and output connectors are also floating inputs and outputs, and maximum rated voltage between the chassis ground and common ground is 42 Vpp (DC + peak AC), which is above your required 25V. This information is found on page 3 of the AFG31000 user manual. Because of these two features, the AFG31000 could be great fit for your application.
AFG31000 Product Page: https://www.tek.com/en/products/signal-generators/arbitrary-function-generator/afg31000#
Tektronix/Keithley typically do not recommend floating an instrument unless necessary, as it can be dangerous if proper precautions are not taken. Before floating an instrument, it is important to understand where all connections lead to and what circuits are energized. They are multiple ways that the AFG could be floated, but the most straight forward is to simply connect the AFG and power supply in series. Most power supplies (PS) utilizes banana jack connections, and most AFGs use a BNC connection. In order to float the AFG, you will need to connect the PS positive terminal to the AFG negative terminal of the male BNC connector. This will require some type of BNC breakout adapter. I have attached an image of a related adapter below. Then connect the AFG positive terminal to the device under test (DUT) , and finally connect the output/ground of the DUT to the PS negative terminal.
BNC Adapter - Male to Double Binding Post
Tektronix actually has an Application Note that discusses floating measurements. This document is more oriented towards oscilloscope use, but you may find concepts and tips within the document helpful.
Fundamentals of Floating Measurements and Isolated Input Oscilloscopes: https://download.tek.com/document/3AW_19134_2_MR_Letter.pdf
